Plex channel on the roku is great for streaming media to your tv. It won't let you do the games thing though.
For my money, I enjoyed turning my pc into a dedicated media box. but I warn you now, if you go down that road you will never stop. It will be an always constant project. not because it's not functional, but because you'll want MORE out of it.
I had mine with my entire DVD collection in full disk images, 6 tv tuners (4 off air HiDef and 2 analog cable), 37 days worth of flac ripped music library, and all your normal gaming and pc functions. And there was always a tweak, also some little more that can be gotten out of it. The ex wife would get frustrated because I was always changing things. It was too "complicated" but oh heaven if it ever broke did she get bent out of shape because she had to go back to "normal" tv.
It's a real love/hate relationship.
For simplistic purpose though, just to have your pc functions on your tv: a dual output video card that you can run one dvi to hdmi out to the tv, and one to the second monitor, wireless keyboard and mouse, and boom, you've got function.